The eye is an organ that focuses incoming light waves onto sensitive nerves. These nerves transmit this information to certain parts of the brain, which in turn interprets them to give the sensation of sight. The act of "seeing" is truly a complex process. Basically, light waves coming into the eye must be bent, i.e. refracted in such a way that they come to a point that focuses on the nerves (retina) at the back of the eye. The two main tissues that bend incoming light are the cornea and the lens. Depending on the length of the eye, the cornea and lens may need to supply different amounts of focusing power. The total amount of power needed for the eye to focus light may be provided in different ratios. If the power of the cornea and the lens is not exactly correlated to the length of the eye, one is said to have a refractive error. The "big three" refractive errors are myopia, hyperopia and astigmatism.
Myopia (from the Greek "shut eyes", referring to the squinting of nearsighted people to see better for distance) is the most common refractive error. In the United States, about seventy million people have myopia and on a worldwide basis myopia affects about one quarter of the total population. Unaided by glasses or contacts, myopic or nearsighted people can see near but not far, since the nearsighted eye is naturally focused at close range. Myopia seems to be on the rise, though it is a mystery why. It is believed to be a result of heredity.
Another common refractive error is hyperopia, or farsightedness, which is the ability to see relatively better at a distance than close up. It occurs when the eye is too short. The lens and cornea are not powerful enough to converge light rays from close objects onto the retina. Even when looking into the distance, a farsighted person not wearing glasses must make a constant effort to focus.
A third refractive error is astigmatism. Astigmatism, from the Greek a, meaning "without", and stigma, meaning "point", results when the image is not focused to a point at all, usually because of an irregularly shaped cornea. When the cornea is so shaped, the rays of light from an object are bent irregularly and, thus, can't be sharply focused for either near or distance. Astigmatism may occur in an otherwise normal eye (simple astigmatism) or in a myopic or hyperopic one (compound astigmatism). Occasionally, the irregular curvature of the cornea may be such that in one direction, for example, vertical, the eye may be nearsighted and in the opposite, i.e. horizontal, axis, the eye may be farsighted. This is called mixed astigmatism.
The most common cause of astigmatism is a cornea that is oval in shape rather than round. A football standing on one end is an exaggerated example of an oval-shaped cornea. The flatter curve of the football from one tip to the other would refract or bend light less than would the steeper curve around the middle of the football. The two main curves of an oval refracting surface, such as an astigmatic cornea, are called meridians, and 90.degree. usually separates the flattest from the steepest meridian. The degree to which the cornea is oval is microscopic.
For a person's vision to be clear without glasses or contact lenses, three structures in the eye have to work together almost perfectly: the cornea, the lens and the retina. The main function of the cornea and lens, as pointed out above, is to bend or refract light from objects so that the image comes to a pinpoint focus and falls on the fovea, the center of the macula. The macula is the critical part of the retina which gives a person sharp, clear vision. Whether light rays enter the eye parallel, as they do from distant objects, or non-parallel (divergent) as they do from near objects, it is up to the cornea and lens to correctly refract them so that the light rays focus precisely on the retina. If light is not brought to a point focus on the retina, vision will be imperfect because of the refractive error of the eye.
Refractive errors are measured in diopters, the unit of measurement of the refractive or light-bending power of the eye. Technically, one diopter is equal to the refractive power of a lens that has a focal length of one meter. A 2-diopter lens will focus parallel rays of light to a point one-half meter away, while a 4-diopter lens will bring light to a focus one-quarter meter (almost ten inches) away. The cornea has 40 diopters or two thirds of the eye's 60 diopters, while the refractive power of the lens is 20 diopters.
The major variables in the eye's exquisite and often imprecise refraction system are the length of the eye and corneal curvature. More accurately, it is the relationship or balance between the axial length, as the length of the eye is called, and the refractive power of the cornea and lens. In general mild to moderate refractive errors occur because of an imbalance between corneal curvature and axial length rather than a true abnormality. The more severe the refractive error, the longer (in myopia) or shorter (in hyperopia) is the length of the eye. Moreover, even if the length of the eye is "normal", a severe refractive error can still occur if the cornea is too steeply curved (in myopia) or too flat (in hyperopia). Normally, the distance from the front to the back of the eye is roughly 24 millimeters (almost an inch). When that distance, the axial length, is longer than normal and does not correlate with the refractive power of the lens and cornea, a refractive error results, i.e., myopia. Similarly, when the axial length is shorter than normal, hyperopia results.
If the eye is too long, light rays refracted or bent by the cornea and lens come to a focus in front of the retina and one is myopic (nearsighted). Similarly, if the cornea is curved too steeply for the length of the eye, a refractive error (hyperopia) will result. An understanding of the sensitive relationship between refractive power of the cornea and lens, and the length of the eye is important in understanding the present invention. The more curved the cornea, the more it will bend light rays and the further in front of the retina they will be focused. The further this focal point lands in front or behind of the retina, the more severe the refractive error.
In summary, the fault in the myopic and the hyperopic eye is that the distance between the front and back of the eye is too long or too short compared to the curvature of the cornea. The fault in the astigmatic eye is that the cornea is irregularly curved, i.e., aspherical shaped, causing light rays to focus in two places. Both the eye distance and corneal curvature increase as people age, commonly causing increased levels of myopia. Glasses and contact lenses compensate for this disparity by altering the way light is bent as it enters the eye. Glasses and contact lenses do not cure refractive errors, i.e., myopia, hyperopia, and astigmatism. They only compensate for these conditions. To attempt to cure myopia, hyperopia, or astigmatism one must change either the length of the eye or the curvature or shape of the cornea. The device and method of the present invention take the latter route, namely, altering the curvature and/or shape of the cornea.
The general concept of making fixed changes in the corneal curvature of the eye is not new. A. E. Reynolds in the U.S. Pat. No. 4,452,235, for example, describes and claims a keratorefractive technique involving a method and apparatus for changing the shape of the optical zone of the cornea to correct refractive error. His method comprises inserting one end of a split ring shaped dissecting member into the stroma of the cornea, moving the member in an arcuate path around the cornea, releasably attaching one end of a split ring shaped adjusting member to one end of the end of the dissecting member, reversibly moving the dissecting member about the path pulling the adjusting member about the path, withdrawing the dissecting member, adjusting the ends of the split ring shaped adjusting member and fixedly attaching the end by gluing to maintain the desired topographical shape of the cornea.
To eliminate some of the complexity of the Reynolds procedure and to facilitate the adjustment of the diameter of the ring inserted in the stroma of the eye, Applicant has found the use of a turnbuckle arrangement most satisfactory. Such an arrangement is not only used to advantage in adjusting the ring size until the corneal topography approximates that of the indication of a desired topography but also to fixedly position the ends of the adjusting ring to the desired topographical shape of the cornea. The need for gluing is eliminated.